Electronic musical instrument



March 24, 1959 T. J. GEORGE 2,879,388

ELECTRONIC MUSICAL INSTRUMENT Filed Aug. 25, 1955 AMPLIFIER INVENTOR.

United States Patent ELECTRONIC MUSICAL INSTRUMENT Thomas J. George, Los Angeles, Calif.

Application August 23, 1955, Serial No. 530,001

8 Claims. (Cl. 250-36) The present invention relates in general to electronic musical instruments, and in particular to an audio oscillator tone generating circuit which is adapted to play selectively, any one of a plurality of single musical frequencies.

Such a circuit is desirable for purposes of economy, if the instrument is to be a solo instrument, where it will never be required to play more than one note at a time. There are at least two applications where a musical instrument of this type is useful. One is in a solo instrument intended for example, for playing the melody only, while the accompaniment is played on some other instrument such as the piano or organ. Another application is in an instrument intended to supply the pedal bass notes for an electronic organ.

For either of these cases a single oscillator according to the present disclosure, can be designed to operate over a frequency range of at least an octave. A plurality of switches operable by means of the keyboard of the instrument, are connected in a sequence circuit, making possible the selective playing of any one of the required musical notes.

It is required first to provide an audio oscillator having good frequency stability, and approximately constant output voltage over the frequency range required. A second requirement is that of providing an economical and practical, individual tuning means for each of the notes to be generated by the oscillator.

The first requirement is met by the use of the conventional'two stage oscillator, using a parallel tuned circuit. The second requirement is met by the use of inexpensive otentiometers, which serve as the individual tuning means.

It is therefore an object of this invention to provide an economical tuning means in a multi-frequency oscillator circuit.

Anotherobject is to provide a two stage oscillator whose voltage output is reasonably constant over a wide frequency range.

Another object is to provide means for using a variable resistance to vary the effective capacitance in a parallel resonant circuit, without appreciably affecting the Q of the circuit.

Another object is to provide a circuit which employs lower cost capacitors than would otherwise be needed.

Briefly, in accordance with the invention a tone generating circuit for use in an electronic musical instrument is provided in Which an oscillator circuit generates a wave of a selectable frequency in accordance with the amplitude of a wave derived from the oscillator which is appliedto a capacitor connected in a frequency determining circuit. Through the provision of a low impedance source for the wave applied to the capacitor, the Q, i.e. efficiency of the frequency determining circuit remains high. In an exemplary embodiment the capacitor receives a wave ice required of the oscillator. The plate terminal of triode 2 is connected to the grid of triode 1 through the network comprising coupling capacitor 12, resistor 10, and resistor 11 in series, and shunt capacitor 7. A parallel resonant circuit comprising inductor 3 and capacitor 4 the access terminals of which are connected together between the grid terminal of triode 1 and the common ground return. This inductor may be of the iron core type, and should have a Q of 5 or higher. Plate coupling resistors 9 and 13 supply plate voltage to the triodes from the B plus supply. The cathode terminal of triode 1 returns to the common ground return through resistor 5. The cathode terminal of triode 2 returns to ground through a pinrality of parallel connected tuning potentiometers 14,15,

and 16. A sequence switching circuit is shown, comprising three sets of switches. The switch arm 20 of the first switch is connected to the resonant circuit and the grid of triode 1. The switch arm 23 of the second switch is connected to the back contact 21 of the first switch 20. The switch arm 26 of the last switch is connected to the back contact 24 of the second switch 23. The last switch 26 has no back contact. The back contact of each switch is normally closed to the switch arm. Each switch has.

a normally open front contact. Front contact 22 is associated with switch 20, contact 25 with switch 23 and contact 27 with switch 26. Tuning capacitor 17 is connected from contact 22 to the movable c ontactor of potentiometer 14, capacitor 18 from contact 25 topo tentiometer 15, and capacitor 19 from contact 27 to porf tentiometer 16.

Coupling capacitor 28 and resistor 29 provide means for taking output signal voltage from the oscillator circuit. Switches 30, 31, and 32 are arranged to selectivelyv connect the output voltage at capacitor 28 to an amplifier 33. The output of the amplifier is connected to loud speaker 34. Switches 30, 31, and 32 are all wired in parallel so that the closure of any switch connects the oscillator output signal to the amplifier. These switches are mechanically arranged to be operated simultaneously with switches 20, 23, and 26, by means of the playing keys of the instrument. Thus switch 30 operates with switch 20, switch 31 with switch 23, and switch 32 with switch 26.

Although only three sets of switches and tuning components are illustrated, it should be understood that twelve sets, together with their appropriate playing keys would be provided, if a range of one chromatic octave is required. I

The circuit operation is as follows. The signal from the plate of the first triode overdrives the grid of triode 2, so that triode 2 is limiting. Reasonable changes in the output voltage from triode 1 will therefore cause no change in the output voltage from triode 2. The signal In this arrangement the plate terminal of triode 1 is connected" to J. the grid of triode 2 through coupling capacitor 6 and re: sistor 8. The reactance of capacitor 6 should be smaller: than the resistance of resistor 8 at the lowest frequency voltage from. triode '2 is brought through blocking con denser 12 and resistors 10 and 11, back to the grid of triode 1, and the tuned resonant circuit. The circuit oscillates at the resonant frequency of the inductor 3 and the capacitor 4 in parallel. Assum ng in one case that the capacitance of condenser 7 is zero, the values of resistors 10 and 11 are selected to considerably attenuate the signal voltage appearing at the grid of triode 1. Triode 1 must operate linearly and without distortion. The amplitude of the feedback signal voltage, and the cathode resistor are selected to meet this requirement. Their values are not critical. The circuit thus oscillates even though no playing keys are depressed, but since none of the switches 30, 31, or 32 are closed, no signal reaches the amplifier and loudspeaker.

The operation when a, playing key is depressed is as follows. A set. of switches is operated by the key, as an example switches, 20 and 30.. Switch arm it) breaks froni contact, 21 and makes. to contact 22, thus connectcapacitor 17 to the resonant circuit. This lowers the frequency of oscillation to a new value, which is determined by the apparent, capacitance of condenser 17 in parallel with the resonant circuit. The potentiometer 14 provides the tuning means for adjusting the apparent capacitance of condenser 17. When the movable contactor arm of the potentiometer tuning control is turned to its limit at the ground end of rotation, the apparent capacitance of capacitor 17 is its actual capacitance. Whenv the contactor arm is turned to its opposite limit at the cathode end of rotation, the apparent capacitance of condenser 17 is considerably greater than its actual value, and the frequency of oscillation is lowered still further. A gradual change in capacitance and frequency occurs as the tuning control 14 is rotated from one limit to the other. An elfective and economical tuning means is thus provided for the oscillator.

There is a sequence of operation of switches 20 and 30, when their associated playing key is depressed. Switch arm 20 closes to contact 22 before switch 30 closes. This is required so that the oscillator will have changed to its new frequency before the output signal is connected to the amplifier and loudspeaker. The reverse order holds when the key is released, and therefore the only pitch heard from the speaker is that selected by the tuning adjustment of potentiometer 14.

The sameprinciple of operation applies to the other switching and. tuning circuits; in the sequence. Thus if'the key associated with switches, 23 and 31 is depressed, the only pitchheard-from the speaker will be that which is determined by the value o f cap'acitor 18 and the adjustment 'of' tuning control 15. Switches 20, 23, and 26 arev connected in sequence so that if two keys are depressed simultaneously, the correct pitch. of only one of the notes will be heard from the speaker. Thus for example, if switches 20 and23 are both operated, only the pitch controlled by switch 20 will be heard, because 'switch 'arinztl in operating, moves away from back contact 21. This opens the circuit to switch 23, thereby making it ineffective.

The values of capacitors 17, 18, and 19 should be selected so that with their respective tuning controls set "to approximately mid-rotation, each associated playing key will play one of three selected consecutive chro matic notes. Thus "with twelve sets of keys, switches, and tuning means, one oscillator may be made to generate selectively, any one of twelve individually tunable chromatic notes.

Tov explain the manner in which the tuning means operates, consider that switch zll is operated, and that at a given instant in a cycle of oscillation, the grid end of, the resonant circuit is moving in a' positive direction, relative to the grounded end. The positive swing is charging capacitor 4 at a rate determined by itscapacitance value. This is also true of capacitor 17, if

tuning control 14 is set at the grounded limit of its roration. But assume that the tuning control is set to the opposite limit. Capacitor 17 now connects to the cathode of triode 2 instead of to ground. At the given instant, since the grid of triode 1 is swinging positive, its plate is swinging negatively. Therefore the grid and cathode of triode 2 are also swinging negatively in the fashion of a cathode follower. The voltage change at the cathode of triode 2 is thus in the opposite phase to that at the grid of triode 1, and is of considerably greater amplitude due to the voltage gain of triode 1. The rate at which capacitor 17 is now charging is thus considerably increased. Similarly its "apparent capacitance is considerably increased. The frequency of oscillation is therefore lowered when the tuning control is set to the cathode limit of rotation, since the apparent capacitance across the tuned circuit has been increased. This is thereforeth'e low frequency limit of the tuning control. As the-control is rotated toward its opposite limit the frequency is gradually increased. The ground limit of rotation is therefore the high frequency limit of the tuning control. Using 'the circuit constants given herein, full rotation of the tuning control will provide approximately a twenty percent frequency change. H

The circuit of Figure l effectively multiplies the actual value of capacitor 17 by a factor of approximately 6, when using 615 tubes. This makes possible the use of lower cost condensers, since the cost varies with the capacitance value. I

It should be understood that while two separate triode tubes are shown in the oscillator of the drawing, a single duo-triode such as the 6SN7 will operate equally well. Other suitable oscillator circuits may be. provided by one or more electron tubes or transistors suitably connected.

In order to avoid introducing too much resistance across the tuned circuit and thereby lowering its Q, when control 14 is set near its mid-position, the resistance of the control should be kept reasonably low. If there are twelve controls connected in parallel, their combined resistance value must also serve as an adequate cathode resistance for triode 2. If the twelve controls each have a resistance value of 25,000 ohms, their combined resistance is 2,100 ohms which is a satisfactory cathode resistance for triode 2. It will be found that this value of control resistance will notv atfect the Q of the tuned circuit to a harmful degree. Accordingly, a feature of substantial advantage of my invention is the control of the frequency of an oscillator without excessively loading or lowering the Q of the frequency determining circuit.

As previously stated-triode 2 is over-driven so that it is distorting. The output signal amplitude therefore does not change perceptibly over the full range of an octave. Also this signal is rich in harmonics, and is identified herein as reed type tone color.

If itis desired to take flute tone also from this oscil lator, a connection can be made to point 35. Another set of switches (not shown) similar to those at 30, 31, and 32 would be required to conduct the flute tone signal from point 35 to the amplifier. In order to hold the flute signal amplitude constant over an octave range, a shunt capacitor 7 may be utilized. This capacitor may be selected, together with the values of resistors 10 and 11, to form a low pass filter, so that the feedback signal voltage appearing at the grid of triode 1 remains at essentially constant amplitude over an octave. range. Without the low pass filter,the feedback signal voltage appearing acrossthe tuned circuit rises 'as its impedance rises, when thefrequency is increased. '-Ihis capacitor thus serves also to eliminate the posssibility of overdriving triode 1 atthe higher frequenciesof oscillation. The phase shift introduced by capacitor 7 does not seem to be detrimental to the frequency stability of the oscillator.

Suitable values for circuit components bearing the following symbols are as follows:

1-615 tube 3-90 henries 4-.005 mfd.

56.8K ohms.

6-.047 mfd.

7-.01 when used.

8-330K ohms.

9100K ohms.

10-11560K ohms when cap. 7 is used. Otherwise 1.5 meg. total.

12.047 mfd.

13-56K ohms.

15-- K ohms each when osc. range is one octave.

17-.O09 mfd., -62'cycles.

18.01 mfd., 58 cycles.

19.012 mfd., 55 cycles.

28-.047 mfd.

29-220K ohms.

I am aware of many applications of the oscillators in the prior art, and of the use of feedback in numerous forms, and of the use of a variable resistor in series with a capacitor for phase and frequency adjustment, but I am not aware of the prior combination of an oscillator whose frequency may be selectively adjusted using fixed capacitors and adjustable resistors, in a circuit which does not lower the Q of the oscillator circuit.

Although for the purpose of illustrating and describing the invention a certain preferred embodiment has been shown, it will be apparent to those skilled in the art that various modifications can be made, such as the use of other oscillator circuits, without departing from the basic principles of the invention, I therefore do not wish to be limited to the precise details set forth herein, but desire to include within the scope of the invention those modified forms by means of which similar results may be obtained in substantially the same way. For example, throughout the specification and claims, various current flow control devices have been designated as electron tubes including a plate, grid and cathode. It is intended that the designation include any equivalent arrangements such as a transistor circuit in which there is included either a junction or point contact current flow control device having a current collector, a current emitter and a control element such as a base.

Having thus described my invention, I claim as follows:

1. In an electronic musical instrument, a two stage oscillator comprising a first triode and a second triode, coupling means connected from the plate of said first triode to the grid of said second triode, coupling means including a voltage attenuator connected between the plate of said second triode and the grid of said first triode, a parallel resonant circuit connected between the grid and cathode of said first triode, a key operated switch having two terminals, a variable resistor, a fixed tuning capacitor, said variable resistor connected between the cathode of said second triode and one terminal of said resonant circuit, a connection between the other terminal of said resonant circuit and one terminal of said switch, said tuning capacitor connected between the other terrnnial of said switch and the movable contactor of said variable resistor.

2. In an electronic musical instrument a two stage oscillator, comprising a first triode and a second triode, circuit connections between the elements of said triodes for causing oscillation, a parallel resonant circuit for determining the frequency of oscillation, said parallel resonant circuit connected between the grid and cathode as'raass' 6 of said first triode, a switch having two terminalna tuning capacitor, a variable resistor, said variable resistor connected between the cathode of said second triode and one terminal of said resonant circuit, a connection from the other terminal of said resonant circuit to one terminal of said switch, a connection from theother terminal of said switch to one terminal of said tuning capacitor, a connection from the other terminal of said tuning capacitor to the movable contactor of said variable resistor, said variable resistor serving as a means for adjusting the frequency of oscillation by altering the apparent capacitance of said tuning capacitor.

3. In an electronic musical instrument, the combination comprising a two stage oscillator adapted to operate over a range of audio frequencies, including a parallel resonant circuit, a first vacuum tube, a second vacuum tube, circuit coupling means interconnecting the ele-. ments of said vacuum tubes and said parallel resonant circuit to cause oscillation at a frequency determined by said resonant circuit, a plurality of capacitors arranged to be connected to said resonant circuit, a plural-. ity of switches arranged for selective operation to connect a selected one of said capacitors to said resonant circuit to lower the frequency thereof, a plurality of parallel connected variable resistors connected between the cathode of one of said vacuum tubes and one terminal of said resonant circuit, each of said variable resistors being associated with one of said capacitors whereby it may operate as a frequency adjusting means.

4. In an electronic musical instrument a two stage oscillator comprising a first triode and a second triode, a parallel resonant circuit connected between the grid and cathode of said first triode, circuit means comprising a resistance capacitance filter for causing said first triode to amplify in a linear manner, circuit means for causing said second triode to amplify in a nonlinear manner, a switch having two terminals, a fixed tuning capacitor, a variable resistor, said variable resistor interconnecting the cathode of said second triode and one terminal of said parallel resonant circuit, one terminal of said switch connected to the grid of said first triode, the other terminal of said switch connected to one terminal of said fixed tuning capacitor, the other terminal of said fixed tuning capacitor connected to said variable resistor, said variable resistor operating as a means of adjusting the apparent capacitance of said fixed tuning capacitor, an output amplifier, circuit means comprising a coupling capacitor and a switch for connecting the anode of said second triode to said output amplifier.

5. In an electronic musical instrument, the combination of an oscillator having a frequency determining circuit including an inductance, a capacitor, a resistor connected serially with the capacitor directly across the inductance, means applying a wave derived from the oscillator to the resistor for charging and discharging the capacitor to present an effective capacitance to the frequency determining circuit to control the frequency of oscillation of the oscillator, and means varying the amplitude of the wave applied to the capacitor to change the frequency of oscillation.

6. Apparatus in accordance with claim 5 including a plurality of key actuated switches coupled to the amplitude varying means to cause the oscillator to generate a wave of a selected frequency in accordance with the actuation of a single switch.

7. A controlled oscillator circuit for use in a manually controlled electronic musical instrument, the operating frequency of said oscillator to be manually controllable over a frequency range corresponding to separate notes of a musical scale, including the combination of an oscillator system including a first amplifier device and having as a frequency determining element thereof a resonant circuit comprising an inductive element and a capacitive element, each of said elements having two access termiuals, each access terminal of said inductive element causes being operatively coupled to a respectively difierent access'trminal of said'capacitive'element, asecond'afi'iplifier device having first, second and "third operatin'gterininal's corresponding in function to the "control grid, cathodean'd anode of a vacuum tube amplifier device, a relatively high impedance input circuit operatively connected between said first and second operating terminals, a'rela'tivelylow impedance output circuit means operatively connected between said second and third operating terminals, a relatively low value output resistance means connected in common to both said input and output circuits for pro viding a relatively low impedance source of signal if -'t'io'n's corresponding to signals appearing in said input circuit, means connecting said 'output'r'e'sistanc'e'me'ans in series with said inductive and capacitive elements -of "said resonant circuit, means operatively cou ling signalsfrom said oscillator to said input circuit to develop a frequency controlling feedback signal across said resistance means and Withins'aid resonant circuit, and means for manually controlling the magnitude of said feedback signal applied to said resonant circuit to permit manual adjustment of said oscillator operating frequency. 1

8. In a controlled oscillator circuit for use in 'a menually controlled electronic musical instrument, the operating frequency of said oscillator to be manually c'ontrollable over a frequency range corresponding to separate notes of a musical scale, the combination of an oscillator system including a first amplifier device and having as a frequency determining element thereof an inductor, a capacitor and a relatively low value resistor connected serially with the capacitor directlyacro'ss the inductor, a

"second 'ainplifir device having first, second and "third opdevice, a relatively high impedance input circuit "operatively connected between said first and secondoperating terminals, a relatively low impedance output circuit means operatively connected between said second and third operating terminals, means connecting said resistor in common with both said input and output means for providing a relatively low impedance source of signal variations corresponding to signals appearing in said input circuit, means operat'ively coupling signals from-said oscillator to said input circuit to develop a frequency controlling feedback signal across said resistor and in series with said capacitor, and means for manually controlling the magnitude of said feedback signal to permit manual-adjustment of said oscillator operating-frequency.

References Cited in the fil'e of this patent UNITED STATES PATENTS 

